The pre-filters (sediment and carbon) do not normally remove TDS (Total Dissolved Solids). The sediment and carbon cartridges are there for the removal of particulate and chemical matter such as rust, sand and chlorine.
The RO process by itself cannot remove all of the TDS, but passing the water through a DI (mixed bed resin) cartridge after going through the membrane removes any remaining TDS. The DI lifespan depends on what the TDS feeding it is like. This is why the RO process is recommended before a DI filter.
The DI (or Mixed Bed Resin) cartridge is made with anion and cation resins for ion removal.